Application is made for funds to purchase one low dose-rate gamma-irradiator and one high dose-rate gamma-irradiator to create a central, shared irradiation facility for use by 6 investigators currently funded by 8 N.I.H. grants, and for use by additional N.I.H. - supported investigators in the future. The 2 instruments will replace the currently available, outmoded irradiator that does not deliver high enough doses of gamma- radiation to large enough numbers of animals at any one time to be optimally useful for studies requiring suppression of immune responses, or inactivation of microorganisms and mammalian cells. Gamma-irradiation is an essential tool in all research projects at the Trudeau Institute. Low doses of precisely monitored, whole body gamma-radiation need to be administered to large numbers of laboratory mice and body gamma-radiation need to be administered to large numbers of laboratory mice and rats for analysis of anti-tumor, anti-allograft and anti-microbial immune responses. On the other hand, high doses of gamma- radiation are needed to efficiently inactivate syngeneic and allogeneic tumor cells and microorganisms for the purpose of immunization.